<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    .button{
      position: relative;
      appearance: none;
      background: #f72359;
      padding: 1em 2em;
      border: none;
      color: white;
      font-size: 1.2em;
      cursor: pointer;
      outline: none;
      overflow: hidden;
      border-radius: 100px;
    }
    span {
      position: relative;
    }
    .button::before {
    --size: 0;  

    content: '';
    position: absolute;
    left: var(--x);
    top: var(--y);
    width: var(--size);
    height: var(--size);
    background: radial-gradient(circle closest-side, #4405f7, transparent);
    transform: translate(-50%, -50%);
    transition: width .2s ease, height .2s ease;
  }

  .button:hover::before {
    --size: 400px;
  }
  </style>
</head>
<body>
    <button class="button"><span>Hello World</span></button>
</body>
<script>
  var oButton = document.querySelector('.button')
  oButton.onmousemove = (e) => {
    const x = e.pageX - e.target.offsetLeft
    const y = e.pageY - e.target.offsetTop
    e.target.style.setProperty('--x', `${x}px`)
    e.target.style.setProperty('--y', `${y}px`)
  }


</script>
</html><!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    .button{
      position: relative;
      appearance: none;
      background: #f72359;
      padding: 1em 2em;
      border: none;
      color: white;
      font-size: 1.2em;
      cursor: pointer;
      outline: none;
      overflow: hidden;
      border-radius: 100px;
    }
    span {
      position: relative;
    }
    .button::before {
    --size: 0;  

    content: '';
    position: absolute;
    left: var(--x);
    top: var(--y);
    width: var(--size);
    height: var(--size);
    background: radial-gradient(circle closest-side, #4405f7, transparent);
    transform: translate(-50%, -50%);
    transition: width .2s ease, height .2s ease;
  }

  .button:hover::before {
    --size: 400px;
  }
  </style>
</head>
<body>
    <button class="button"><span>Hello World</span></button>
</body>
<script>
  var oButton = document.querySelector('.button')
  oButton.onmousemove = (e) => {
    const x = e.pageX - e.target.offsetLeft
    const y = e.pageY - e.target.offsetTop
    e.target.style.setProperty('--x', `${x}px`)
    e.target.style.setProperty('--y', `${y}px`)
  }


</script>
</html>